(1) Technical Field
This invention applies generally to methods of photo-printing microcircuit patterns on photosensitive coated substrates used in the manufacture of semiconductor slices and more particularity, to a method and apparatus for removing a pellicle frame from a surface of a photomask consisting of an array of enlarged circuit patterns.
(2) Description of the Prior Art
The following three documents relate to various methods dealing with pellicle mounting apparatus.
U.S. Pat. No. 4,637,713 issued Jan. 20, 1987 to Shulenberger et al, discloses a pellicle mounting apparatus having a pair of pellicle holder assemblies positioned on either side of a photomask holder assembly.
U.S. Pat. No. 5,311,250 issued May 10, 1994 to Suzuki et al, shows another pellicle mounting apparatus.
U.S. Pat. No. 5,422,704 issued Jun. 6, 1995 to D. Sego discloses a pellicle frame.
A wafer for the manufacture of a semi-conductive elements is subjected to numerous sequenced operations which enable the fabrication of all its circuit elements. The use of a photo resist material in conjunction with photograhic masks provides the means to transfer the various pattern layers to the semiconductor wafer. The photo mask carries an enlarged pattern which is optically reduced upon each wafer's surface by projection printing. The layers upon which these images are to be reproduced are either a semiconductor, an oxide on a semiconductor, or a metal film covering the semiconductor.
The image formed in the photo resist can not be better than the photomask used to determine the exposed and unexposed areas. The most important characteristics that the image must possess are; dimensional correctness, uniform density in both clear and opaque areas, and sharp definition between clear and opaque areas.
In most applications, thousands of images are formed on a single photomask. A sequenced series of photomasks is required to complete a given semi-conductive device. Each mask controls the exposure of window patterns on wafers coated with photo sensitive resist material. This resist material is processed (developed) to wash away the exposed resist which permits access through the windows for depositing, implanting, or etching a variety of materials.
Multiple mask images which make up a mask series are produced with a defined, closely controlled spacing. In most semiconductor manufacturing operations, a series of photomasks are required to produce finished semi-conductive devices. Since the series of masks are used sequentially, mask alignment and the center to center spacing between features produced by successive masks must conform to preceding mask images to form a coherent array of materials in the layers of the devices being built.
It is well known that during the manufacture of semiconductive devices the goal is to achieve defect free exposures of circuit patterns. As integrated circuits evolved from small scale integration to very large scale integration the need for ultra clean manufacturing space became increasingly critical. As an example, a single airborne particle landing on the photomask surface during exposure can ruin the circuit exposed on the wafer below it.
To help solve this problem, the photolithographic industry developed pellicles to intercept particulates and protect the surfaces of photomasks against all forms of contamination. Pellicles include a thin, transparent membrane stretched across a frame. The frame holds the membrane in tension and keeps it away from the mask surface by the thickness of the frame. An adhesive is used to bond the frame to a surface of the photomask.
Because of the obvious inherent simplicity of this device, it has found use over a very wide range of the electromagnetic spectrum. The pellicle concept has been used in the optical industry for many years. A transparent membrane would have the effect of increasing the number of wavelengths in an optical path, or as a beam splitter in interferometry. Its application has been extended to photolithography by intercepting dust particles and other contaminents from reaching the focal plane of photomasks.
Many difficulties have been fronted in properly removing pellicle assemblies from photomasks. This procedure is necessary when cleaning or replacing damaged pellicle membranes. Presently, this job is done manually with the aid of simple hand tools and a stereomicroscope. Techniques for removing pellicle assemblies from photomasks vary widely. Each technician developes their own procecure and craftiness therefore, at times, cause costly miscues.